Pirkle phase

There is a wide variety of commercially available chiral stationary phases and mobile phase additives.32 34 Preparative scale separations have been performed on the gram scale.32 Many stationary phases are based on chiral polymers such as cellulose or methacrylate, proteins such as human serum albumin or acid glycoprotein, Pirkle-type phases (often based on amino acids), or cyclodextrins. A typical application of a Pirkle phase column was the use of a N-(3,5-dinitrobenzyl)-a-amino phosphonate to synthesize several functionalized chiral stationary phases to separate enantiomers of... 

Chiral stationary phases for the separation of enantiomers (optically active isomers) are becoming increasingly important. Among the first types to be synthesized were chiral amino acids ionically or covalently bound to amino-propyl silica and named Pirkle phases after their originator. The ionic form is susceptable to hydrolysis and can be used only in normal phase HPLC whereas the more stable covalent type can be used in reverse phase separations but is less stereoselective. Polymeric phases based on chiral peptides such as bovine serum albumin or a -acid glycoproteins bonded to... 

More recent developments in the field of the Pirkle-type CSPs are the mixed r-donor/ r-acceptor phases such as the Whelk-Of and the Whelk-02 phases.The Whelk-Of is useful for the separation of underiva-tized enantiomers from a number of families, including amides, epoxides, esters, ureas, carbamates, ethers, aziridines, phosphonates, aldehydes, ketones, carboxylic acids, alcohols and non-steroidal anti-inflammatory drugs.It has been used for the separation of warfarin, aryl-amides,aryl-epoxides and aryl-sulphoxides. The phase has broader applicability than the original Pirkle phases. The broad versatility observed on this phase compares with the polysaccharide-derived CSPs... 

There are numerous chiral stationary phases available commercially, which is a reflection of how difficult chiral separations can be and there is no universal phase which will separate all types of enantiomeric pair. Perhaps the most versatile phases are the Pirkle phases, which are based on an amino acid linked to aminopropyl silica gel via its carboxyl group and via its amino group to (a-naphthyl)ethylamine in the process of the condensation a substituted urea is generated. There is a range of these type of phases. As can be seen in Figure 12.23, the interactions with phase are complex but are essentially related to the three points of contact model. Figure 12.24 shows the separation of the two pairs of enantiomers (RR, SS, and RS, S,R) present in labetalol (see Ch. 2 p. 36) on Chirex 3020. 

Persson and Andersson [65] reviewed the unusual effects in liquid chromatographic separations of enantiomers on chiral stationary phases with emphasis on polysaccharide phases. On protein phases and Pirkle phases, reversal of the elution order between enantiomers due to... 

A. M. Dyas, M. L. Robinson, and A. G. Fell, An evaluation of the structural requirements for the separation of propranolol enantiomers on Pirkle phases following achiral derivatisation, Chromatographia, 30 13 (1990). 

The pioneering work in the area of chiral 77-donor and 77-acceptor phases was done by Pirkle and co-workers,98,99 and the most frequently used 77-acceptor phase is still an (/ )-7V-(3,5-dinitrobenzoyl)phenylglycine phase, the so-called Pirkle phase. With these stationary phases, chiral recognition is based on 77-77 interactions, dipole stacking interactions, and hydrogen bonding.100... 

Mechanistic considerations (e.g., the extensive work published on brush-type phases) or the practitioner s experience might help to select a chiral stationary phase (CSP) for initial work. Scouting for the best CSP/mobile phase combination can be automated by using automated solvent and column switching. More than 100 different CSPs have been reported in the literature to date. Stationary phases for chiral pSFC have been prepared from the chiral pool by modifying small molecules, like amino acids or alkaloids, by the deriva-tization of polymers such as carbohydrates, or by bonding of macrocycles. Also, synthetic selectors such as the brush-type ( Pirkle ) phases, helical poly(meth) acrylates, polysiloxanes and polysiloxane copolymers, and chiral selectors physically coated onto graphite surfaces have been used as stationary phases. 

Enantioselective packings Polar or nonpolar Packings with enantioselective cages or enantioselective surfaces, microcrystalline cellulose triacetate, cellulose ester or cellulose, carbamate/sil-ica composites, optically active poly(acrylamide)/silica composites, chemically modified silicas (Pirkle phases), cydodextrine modified silicas Operated either with normal phase or reversed phase mobile phases... 

Section 7.5 outlined how silica can be derivatized with almost any functional group the resulting monomer structures are known as brushes . The first broadly used and still very important CSP is the brush-type dinitrobenzoylphenylglycine (DNBPG), the first one shown in Table 22.1. According to its inventor, William H. Pirkle, it is often called Pirkle-phase , although a more correct name is Pirkle T because it is not the only one of his phases that is on the market. 

Figure 10.3 Representative bonded chiral phases for HPLC. (I) Pirkle phase (II) N-(3, 5-dinitrobenzoyl) phenylglycine ionically bonded to 3-asinopropylsilanized silica (III) N-n-valeryl-L-valyl-3-aminopropylsilanized silica (IV) R-(-)-2-(2-4-5-7-tetranitro-9-fluorenylideneaminooxy) propionamidepropyl silanized silica.

For applications such as enantiopohshing or chiral separations, MIPs are said to offer low capacity. This is true when comparing MIPs with sorbents that rely on an interaction with a surface or a surface modified with a chiral selector (e.g. Pirkle phases). On the other hand, MIPs are at least competitive if one compares their atom economy with that of enzymes or antibodies adding up the molar mass of crosslinker and receptor sites per template (between 5-100kDa depending on synthesis recipe) [14]. 

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